Free KCSE Physics Topical Questions KNEC Standards

K.C.S.E Physics Q & A - MODEL 2012PP2QN018

21/7/2020

K.C.S.E Physics Q & A - MODEL 2012PP2QN018

 Figure 18 shows parts of an x-ray tube.

(a) Explain why:
(i) A potential difference is applied to the filament.
(ii) A high potential difference is applied between the cathode and the anode.
(iii) Most of the tube is surrounded by lead.
(b) State how the resulting x-rays are affected by increasing the potential difference between the anode and the cathode.
(c) Light of frequency 7.5 x 10¹⁴ Hz strikes a metal surface whose work function is 4.0 x 10^-19J. Determine the kinetic energy of the emitted photoelectrons. (take planks constant h = 6.63 x 10^-34 Js)

answers

(a) (i) To produce electrons; by thermionic emission;
(ii) To accelerate the electrons to give them enough K.E. t produce X-rays at the anode;
(iii) To absorb stray X-rays, thus protecting the operator from those rays;
(b) Increases K.E. of electrons and hence causes X-rays of higher frequency;
OR
X - ray are more penetrative
X - rays of shorter wavelength.

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K.C.S.E Physics Q & A - MODEL 2012PP2QN017

21/7/2020

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K.C.S.E Physics Q & A - MODEL 2012PP2QN017

(a) Figure 16, shows a graph of magnification against object distance, for an object placed infront of a lens of focal length 20cm.

Using the graph;
(î) State the effect on the size of the image as the object distance is increased from 25cm.
(ii) Determine the distance between the object and the lens when the image is the same size as the object.
(iii) Determine the image distance when the object distance is 25 cm.
(b) Figure 17 shows an object O placed in front of a converging mirror of focal length 15cm.

Draw on the figure a ray diagram to locate the image formed.
(c) State why parabolic reflectors are used in car headlights.

answers

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K.C.S.E Physics Q & A - MODEL 2012PP2QN016

21/7/2020

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K.C.S.E Physics Q & A - MODEL 2012PP2QN016

(a) Figure 15, shows two coils A and B placed close to each other. A is connected to a steady D.C supply and a switch, B is connected to a sensitive galvanometer.

(i) The switch is now closed. State the observation made on the galvanometer.
(ii) Explain what would be observed if the switch is then opened.
(b) The primary coil of a transformer has 1000 turns and the secondary coil has 200 turns.
The primary coil is connected to a 240V a.c mains, supply.
(1) Explain how an e.m.f is induced in the secondary coil.
(ii) Determine the secondary voltage.
(iii) Determine the efficiency of the transformer given that the current in the primary coil is 0.20A and in the secondary coil is 0.80A.

answers

(a) (i) Pointer deflects upto a certain; maximum value and then returns to zero;
(ii) There is a deflection in the opposite direction then back to zero; As Flux in A falls, flux in B also falls and causes induced e.m.f in the opposite directions;
(b) (i) Current in the primary is constantly changing its direction; so that the resulting flux (which link coils) is constantly changing its “
direction. Therefore alternating e.m.f is induced in the secondary coil;

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K.C.S.E Physics Q & A - MODEL 2012PP2QN015

21/7/2020

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K.C.S.E Physics Q & A - MODEL 2012PP2QN015

Figure 14, shows a circuit in which a battery, a switch, a bulb, a resistor P, a variable resistor Q, a voltmeter V and two ammeters A1 and A2 of negligible resistance are connected.

P has a resistance of 10Ç2. When the switch is closed A1 reads 0.1OA and the voltmeter reads 1.5V.
(a) Determine;
(i) the current passing through P.
(ii) the resistance of the bulb.
(b) The variable resistor is now adjusted so that a larger current flows through A2.
(i) State how this will affect the resistance of the bulb.
(ii) Explain your answer in b(i).
(c) A house has one 100W bulb, two 60W bulbs and one 30W bulb. Determine the cost of having all the bulbs switched on for 70 hours, given that the cost of electricity is 40 cents per kilo-watt hour.

answers

(b) (i) the resistance of the bulb would increase;
(ii) Current is higher hence increases; temperature increased temperature results in increased resistance;

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K.C.S.E Physics Q & A - MODEL 2012PP2QN014

21/7/2020

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K.C.S.E Physics Q & A - MODEL 2012PP2QN014

(a) Figure 12, shows a displacement - time graph for a progressive wave.

(i) state the amplitude of the wave.
(ii) determine the frequency of the wave.
(iii) given that the velocity of the wave is 20m/s,determine its wavelength.
(b) Figure 13 shows two identical dippers A and B vibrating in water in phase with each other. The dippers have the same constant frequency and amplitude, The wavesproduced are observed along line MN:

It is observed that the amplitudes are maximum at points Q and S, and minimum at points P and R.
(i) explain why the amplitude is maximum at Q.
(ii) state why the amplitude is minimum at R.
(iii) State what would happen if the two dippers had different frequencies.

answer

(b) (i) Waves at Q are in phase q so there is constructive interference,
(ii) Waves are out of phase hence destructive interference,
(iii) Interference pattern would disappear.

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FORM I - PHYSICS SYLLABUS

Introduction to Physics (4 Lessons)
Measurements 1 (12 Lessons)
Force (16 Lessons)
Pressure (24 Lessons)
Particulate Nature of Matter (12 Lessons)
Thermal Expansion (12 Lessons)
Heat Transfer
Rectilinear Propagation of Light and Reflection at Plane Surface (16 Lessons)
Electrostatics (12 Lessons)
Cells and Simple Circuits (12 Lessons)

FORM II - PHYSICS SYLLABUS

Magnetism (12 Lessons)
Measurements II (16 Lessons)
Turning Effect of Force (10 Lessons)
Equilibrium Centre of Gravity (12 Lessons)
Reflection at Curved Surfaces (16 Lessons)
Magnetic Effect of Electric Current (18 Lessons)
Hooke's Law (8 Lessons)
Waves (14 Lessons)
Sound (12 Lessons)
Fluid Flow (14 Lessons)

FORM III - PHYSICS SYLLABUS

Linear Motion (20 Lessons)
Refraction of Light (20 Lessons)
Laws of Motion (15 Lessons)
Energy, Power and Machines (20 Lessons)
Current Electricity (20 Lessons)
Waves II(10 Lessons)
Electrostatics II (15 Lessons)
Heating Effect of Electric Current (10 Lessons)
Quantity of Heat (20 Lessons)
Gas Laws (15 Lessons)

FORM IV - PHYSICS SYLLABUS

Thin Lenses (20 Lessons)
Uniform Circular Motion (10 Lessons)
Floating and Sinking (15 Lessons)
Electromagnetic Spectrum (15 Lessons)
Electromagnetic Induction (20 Lessons)
Mains Electricity (10 Lessons)
Cathode Rays and Cathode Ray Tube (10 Lessons)
X-rays (8 Lessons)
Photoelectric Effect (15 Lessons)
Radioactivity (15 Lessons)
Electronics (10 Lessons)

K.C.S.E Physics Q & A - MODEL 2012PP2QN018

K.C.S.E Physics Q & A - MODEL 2012PP2QN018

K.C.S.E Physics Q & A - MODEL 2012PP2QN017

K.C.S.E Physics Q & A - MODEL 2012PP2QN017

K.C.S.E Physics Q & A - MODEL 2012PP2QN016

K.C.S.E Physics Q & A - MODEL 2012PP2QN016

K.C.S.E Physics Q & A - MODEL 2012PP2QN015

K.C.S.E Physics Q & A - MODEL 2012PP2QN015

K.C.S.E Physics Q & A - MODEL 2012PP2QN014

K.C.S.E Physics Q & A - MODEL 2012PP2QN014

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FORM I - PHYSICS SYLLABUS

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